Set links to :focus-visible instead of :focus

[markconroy]

When you click on a link you get the same styles as the focus styles. This is because clicking a link activates the focus indicator.

What these means for us is when you click on a link, they turn to white, with black text, thick black border and yellow border - like the GOV.UK system.

Now that CSS has focus-visible let's switch to using that instead, so this style is only applied when you focus a link and not when you hover/click it.